1. Field of the Invention
The present invention relates to a radiation detector that detects quantities of radiation such as X-rays transmitted through an object being imaged and a radiation imaging apparatus provided with such a radiation detector to produce images of objects being imaged using data of the radiation detected by the detector, and in particular, to a radiation detector in which a plurality of direct conversion type of modules are arranged which have semiconductor layers to directly convert the radiation to electric signals corresponding in quantities to the radiation or a large-size radiation detector in which a plurality of indirect conversion type of modules are arranged and each module has detections cells which convert the radiation to light and then convert the light to electric signals, and a radiation imaging apparatus equipped with such a radiation detector.
2. Background Art
Techniques for medical modalities of recent years have been developed remarkably. Such medical modalities include medial panoramic imaging apparatuses which can acquire tomographic images along patient's tooth rows using X-ray beams and medical CT scanners which can acquire tomographic images along arbitrary sections at designated portions of patient's bodies.
No less important is the fact that performance improvement in X-ray detectors has contributed significantly to development of such X-ray-related medical modalities. As exemplified by a patent reference 1 for example, such X-ray detectors are known as, what is called, a direct-conversion type of X-ray detector with a semiconductor, in which the detector has a detection layer made of semiconductor such as CdTe so that X-ray beams impinging on this detection layer are directly converted to corresponding electric signals. Meanwhile, so called indirect conversion type X-ray detectors are also in frequent usage, in which the detectors have a scintillator, made of CsI and GOS, which convert radiation to light, and a CCD circuit, photo diode circuit or C-MOS circuit which converts the light to corresponding electric signals.
In these X-ray detectors, the X-ray detection layer of a semiconductor type detector is produced by making an ingot grow and forming and processing the grown ingot. Hence it is difficult to produce, as one element, a detector having a larger detection area. In consideration of this fact, modules of given sizes (for 8 mm×8 mm, which is rectangular) are produced where a given number of pixels (for example 40×40 pixels) are mapped in a two-dimensional array. A plurality of such modules are adjacently located closely with each other in a two-dimensional form or a liner form, so that a structure with adjacent arrangement of the modules is employed. This can provide an X-ray detector having a two- or one-dimensional detection area, in which, in fact, even the one-dimensional detection area still presents a detection width corresponding to one module. In this close arrangement of the modules, it is necessary to provide both high assembly accuracy and wiring spaces to the modules, which requires that there be arranged spaces of a given width between modules (this space is also called “a gap”, of which the width is normally approximately 0.5-5 times that of a single pixel). From a viewpoint of producing compact detectors, these spaces result in dead space.
By the way, in panoramic imaging apparatuses and X-ray CT scanners, X-ray scanning is normally performed parallel with the direction of one of mutually orthogonal coordinate axes along which pixels of an X-ray detector are arranged longitudinally and laterally in the rectangular detection areal. For instance, the lateral axis of the orthogonal coordinate and the scan direction coincide with each other. In this case, there is a gap present between modules, which has a given width. The extending direction of the gap coincides with the scan direction. However, the X-ray detector is moved laterally for scanning, so that the gap acts as a dead zone which cannot pickup X-ray transmittance information of an object being examined.
In addition, the pixels of the X-ray detector include pixels located at the outermost ends of each module, where such outermost pixels tend to exhibit unstable X-ray detection performance. Hence, when X-ray transmission data (i.e., frame data) including data from pixels with no detection and data from pixels with unstable detection are used to reconstruct images, artifacts are caused in the images and/or image information is incomplete in the images, which will result in a reduction of accuracy of image interpretation.
To improve this issue, there have been known X-ray detectors which are disclosed by a patent reference 2 and a non-patent reference 1, in which the detectors have detection surfaces composed of plural rectangular modules arranged obliquely to the scan direction. In such detectors, the longitudinal direction of a gap located between modules, which acts as a dead space, is also oblique to the scan direction, whereby there is no dead zone which cannot detect data.
Additionally, in recent years, use of a photon counting type of X-ray detector has been started, as indicated by patent references 3, 4, and 5, and a non-patent reference 2.